Our Changing Solar System
Recent discoveries have been altering our views of many worlds. Ancient observers knew nothing about heavenly bodies other than their position, brightness, and color. The development of the telescope permitted a huge increase in astronomical knowledge. Until Galileo looked, nobody knew that other planets have moons too or that Venus shows phases. Telescopic observations have added three outer planets and over two thousand asteroids (planetoids) to our lists of the sun’s family in the past 200 years. Within the past generation space probes have brought home to us the true diversity of other worlds and shattered many preconceptions.
The Sun is the star of our solar system: Sol, Apollo, Helios. The source of light and life for the whole system. For millennia, humanity looked at the sun without any understanding of the source of its energy. Just in this century has nuclear physics presented a model to describe the fusion reactions (creation of new elements) that power the sun. In the past decade, observations have been made that are presenting conventional solar models with severe problems. A re-examination of old records has shown that the sun is much more variable than previously believed. A neutrino capture experiment at the bottom of a mine has failed to show the number predicted by current solar theories. Direct observation of the sun has shown that it is pulsating with sound waves. Our picture of the sun (and as a consequence of all stars) may be drastically changed by the time theorists figure out how to account for all of these observations.
Mercury: the closest planet to the sun, the fastest moving planet, the messenger of the gods. Never more than 28° from the sun, never visible except in daylight or twilight—one of the hardest planets to observe. Since the last century, Mercury was thought to always keep the same face turned to the sun just as our moon always faces us. Radar observations in the middle 1960s finally showed that the sidereal day (of 58.65 earth days) is only two thirds the sidereal year (of 87.97 earth days). No longer can we describe Mercury as a tide-locked planet with a perpetual day side, night side, and twilight zone. There is still a tidal coupling, but with a ratio of 3:2 (days:years) instead of 1:1. This gives Mercury a solar day unique in the solar system. One solar day on Mercury is 3 sidereal days or 2 sidereal years long. Between one dawn and the next, Mercury circles the sun twice. On top of that, when Mercury is near perihelion its orbital speed exceeds its rotational speed and the sun goes retrograde in its sky.
The synodic period (the time taken for the sun, another planet, and the earth to return to the same relative positions—conjunction to conjunction) of Mercury is 115.4 earth days.
Physically, Mercury is a small world, not much bigger than our moon. It is heavily cratered and has virtually no atmosphere. Noon temperatures on Mercury get as high as 800° F. Our knowledge of its topography comes from the Mariner 10 flybys 5 years ago.
Venus: the planet that comes closest to earth, our sister planet, the veiled planet, the goddess of love. Nearly the same size as earth and close to us in the solar system, Venus was long thought to be a near twin with excellent chances for life. With its lower atmosphere and surface hidden by clouds, speculation made Venus everything from a worldwide ocean to carboniferous swamps to gigantic forests. It was frequently spoken of as a younger world than earth.
Now we know that Venus is much closer to classical pictures of Hell than of Heaven. The atmosphere has almost no free oxygen—it is mostly carbon dioxide, with clouds of sulfuric acid. The atmospheric pressure at the surface is 100 times that of earth. The temperature on Venus is higher than the noon temperature of Mercury—890° F—and it doesn’t cool off at night.
Venus rotates slowly in a clockwise (retrograde) direction with a sidereal day of 243.1 earth days. Combined with its year of 224.7 earth days, this gives a solar day of 116.8 earth days. The synodic period of Venus is 583.9 earth days.
There are spacecraft investigating Venus right now, so we will soon know much more about its atmosphere and surface. They have already shown us a lot about the global wind patterns on Venus, with gentle breezes at the surface and high winds at higher altitudes. The radar aboard the spacecraft now orbiting Venus is revealing large scale features on the surface. We now know Venus has canyons or rift valleys on a scale at least as large as earth, and an uplifted plateau and mountain range larger than the Tibetan plateau and Himalayas. There are also signs of huge volcanoes, but those identifications are still tentative.
Our moon: Luna, Selene, Diana, the huntress, the light of the night, our nearest neighbor in space. Our moon is a dead world. Galileo observed, contrary to contemporary dogma, that the moon was not a perfectly unblemished sphere. The features of the moon were given terrestrial names—seas and mountains—but are quite unlike what we have on earth. The moons “oceans” are huge craters with layers of lava covering their floors. Most of the surface of the moon is covered with impact craters. Almost the only visible feature due to activity of the moon itself (rather than external impacts) is the lava in the “seas”. The Apollo missions and many other space probes have given us knowledge of the moon impossible to obtain from earth. We know that the visible surface has been almost unchanged for 3 billion years. The moon, and to the best of our knowledge the whole solar system, is 4.6 billion years old, but most of the cratering of the moon occurred in the first billion years. The moon has enough chemical similarities to and differences from the earth to leave the old arguments about whether it formed with the earth or separately still unsettled. The moon is a repository of the past, and the only changes that occur are caused by outside forces.
The moon revolves around the earth in a sidereal month of 27.32 solar earth days. It also rotates on its own axis with the same period, so it always turns the same face to us. The synodic month (new to new) is 29.53 earth days.
The earth: Gaia, Terra, our home. This century has seen our conception of our home change from a static, unchanging world to a world in constant motion. The face of the globe changes as the continents move about on it. Colliding continents produce mountain chains—the Himalayas are the still rising product of the collision of India and Asia. Oceans and seas change their size and shape as the continental plates move—the Mediterranean Sea as we know it is only thousands of years old. Not too many million years ago there was only one continent. The birth world of life is a living world.
Mars: Ares, the god of war, the blood red planet. Where Venus was viewed as earth’s younger sister, Mars was earth’s older brother. (A theory of the solar system’s origin that was once popular included the belief that the planets started out molten. The farther a planet was from the sun, the sooner it could cool and permit life.) We can see the surface of Mars in our telescopes, and it visibly changes with the seasons. A reasonable bit of projection leads to the suggestion that the changes are the life cycles of vegetation. At the limits of resolution of the best telescopes there seem to be lines visible on the surface. An Italian astronomer calls them “canali” (channels) and English speakers think “canals”. Percival Lowell maps vast systems of canals and speculates about the intelligences fighting the drying out of their aging planet. H. G. Wells gives us Martian invaders, and Edgar Rice Burroughs gives us Barsoom. Better telescopes don’t show the canals but the myths persisted until space probes get a closer look.
Mars, the next planet out from the sun after earth, is a small, cold world. It is only a little more than half the diameter of the earth, and circles the sun in 686.93 earth days. The Martian day is less than an hour longer than an earth day. The axis of rotation is tilted 25° from the perpendicular to its orbit, only 2 degrees more than earth’s. Since it is one and a half times the earth’s distance from the sun, it receives less heat and all the season’s are longer and colder than ours. The atmosphere of Mars is only about one hundredth as dense as that of earth, and is mainly carbon dioxide.
Several Mariner flybys showed no canals and many craters—seemingly Mars was another dead world like our moon. Then Mariner 9, an orbiter, showed us a whole new picture of Mars. Parts of it were indeed cratered like the moon, but it also has huge volcanoes, a tremendous canyon, and what looks like evidence of past flooding. The largest volcano in the solar system (that we have seen so far), Olympus Mons would fill the distance between Los Angeles and San Francisco. Valles Marineris, the valley of the Mariners, is a canyon that would stretch the whole width of the continental United States. Mars does things in a big way!
The Viking spacecraft, which are still sending data back from Mars, have given us even more information than the Mariners. The landers looked for any signs of life in the terrestrial mold, and failed to find any. The orbiters have photographed the planet in much greater detail than before, and given us our best looks at the two tiny moons of Mars (Phobos and Deimos—fear and panic). They have seen many more of the features first seen by the Mariners that look so much like channels left by running water. They have finally settled the question of whether the polar caps of Mars are water ice or carbon dioxide ice—they are mostly dry ice, but the residual north polar cap is water ice. Like most scientific investigations, the Viking mission has raised more questions than it has answered, and scientists are already planning the next step.
The asteroids were unknown until the 19th century. Now there are over 2,000 that have been observed long enough to have their orbits established. They are small bodies—the largest known, Ceres, has a diameter of around 1,000 kilometers. Most of the asteroids (or planetoids) orbit between Mars and Jupiter, but at least one, Icarus, gets closer to the sun than Mercury ever does. One of the most recent discoveries, Chiron, which may or may not be an asteroid, orbits between Saturn and Uranus. The asteroids have not yet been visited by any of our space probes, so anything we know about them has been determined with telescopes. (Two of the asteroids which come relatively close to earth have been observed with radar, but none in the main belt.) The physical makeup of the asteroids is believed to be much like that of the meteors that have been analyzed after falling to earth. Most are believed to be irregular in shape, unlike the spherical planets. One of the latest speculations is that some asteroids, small as they are, may have moons. The observations leading to this speculation are still disputed, so we can’t be sure.
Jupiter: Jove, Zeus, the thunderer, top god. Jupiter is the largest of the planets. It emits more energy than it receives from the sun, which only one other planet in the solar system (Saturn) does. A single storm on the surface of its clouds, the great red spot, has been observed ever since we have had telescopes and is larger than the earth. Its satellites make up a whole solar system in miniature. Its magnetic field extends for many millions of kilometers.
We are now beginning to get a clearer picture of the forces driving Jupiter’s weather. The Great Red Spot, known for 300 years, and several smaller white ovals observed for the last 40 years, are all huge vortexes. The alternating bands and zones and regions of turbulent interaction between them are all driven by the energy generated within Jupiter.
Four of Jupiter’s moons, called the Galilean satellites in honor of their discoverer, are as large as or larger than our own moon. Io and Europa are both about the size of our moon and Ganymede and Callisto are both about the size of Mercury. The two Voyager spacecraft which just this year went through the Jovian system have given us our first close looks at these satellite worlds. They are worlds as different as any other worlds in the solar system.
Probably the most spectacular discovery of the mission is the nature of Io. Io was observed to have 8 volcanoes active when Voyager 1 went by. This makes it only the second world in the solar system known to have active volcanoes (earth is the first). Voyager 2 was able to observe 7 of the eight volcanoes when it went by four months after Voyager 1 and found 6 of them still active. There is so much volcanic activity on Io that it is believed to replace its surface completely on a time scale of only thousands or millions of years. The most popular theory for the cause of Io’s vulcanism was proposed just before the flybys. It has been calculated that Io is subject to tidal forces strong enough to raise 300 foot tides in its rocks, producing a lot of heat in the process. The result is the observed volcanoes with plumes hundreds of kilometers high.
Europa, Io’s near twin in size, is drastically different in structure. The surface is almost completely covered with a maze of cracks. There is almost no relief (difference in height). Europa is believed to be a rocky body with a thin (100 kilometer) shell of ice. Only a few impact craters have been spotted, indicating that Europa’s surface is relatively young (several hundred million years old), but still older than the surface of Io with its complete lack of impact craters.
Ganymede offers a mixed picture. There are dark, heavily cratered areas, grooved areas, impact basins, and smooth terrain. There is one series of concentric rings that may be the remnants of a really huge impact. The dark, cratered areas are the oldest and the bright rayed impact basins the youngest terrains. Ganymede also shows evidence of faulting such as we experience in earthquakes.
Callisto may have the oldest surface we have yet observed anywhere in the solar system. It is shoulder to shoulder craters. The cratering is believed to date back to the heavy bombardment experienced by every body in the solar system during the closing phases of the period of planetary formation, making it well over 4 billion years old.
The Voyagers have also discovered a thin ring around the planet Jupiter, making it the third ringed planet in the solar system. Voyager 2 also discovered a small satellite in the outer edge of Jupiter’s ring. The satellite is less than 40 kilometers in diameter and orbits Jupiter in only 7 hours and 8 minutes—the shortest orbital period of any satellite in the solar system. The Voyager flybys are too recent for much of the data they produced to have been absorbed, so we can expect to keep hearing new things about Jupiter and its miniature solar system for some time to come.
Saturn: Chronos, old father time, the father of Jupiter, the ringed planet, the outermost planet known to the ancients. Saturn’s rings, known only for a little over 300 years, are one of the solar system’s more spectacular sights. They are visible in even a small telescope, and make Saturn a favorite subject for photography. Saturn’s largest moon, Titan, is larger than Mercury and actually has an atmosphere.
Pioneer 11 just flew by Saturn on September first of this year, giving us a lot of new information to assimilate. It added a faint outer ring to the list of rings, and confirmed the existence of a gap in the rings tentatively identified from earth. It found that the rings of Saturn act as a very efficient radiation shield, reducing the intensity of charged particles to much more endurable levels (though still fatal to humans). It observed several small moons, at least one of which is new. The orbits of the moon(s) most recently discovered from earth are still uncertain so it will be a while before anyone can say for certain whether some of the moons Pioneer 11 observed are the same ones seen from earth.
Uranus: the father of Saturn, the chaos that came before order. Uranus was the first planet unknown to the ancients to be discovered after the invention of the telescope. It was discovered in 1781 by William Hershel. The relation of Uranus’ axis of rotation to its orbit is perhaps the most unusual known in the solar system. While all the other planets rotate so that the sun will never be too far from their equators, Uranus rotates so that the sun at times shines directly on its poles. Where the tilt of the earth’s axis from the perpendicular to its orbit is only 23.5°, Uranus’ axis is tilted 97°. In effect, Uranus rolls along in its orbit like a ball rather than spinning like a top. The rotation period was revised upward from around 10 hours to around 22 hours within the past generation.
Just two years ago, it became the second ringed planet. Observations as Uranus passed in front of a star led to the discovery of the rings, which have since been confirmed telescopically. Uranus is so distant we know very little about it, and no space probes from earth have yet visited it. Voyager 2 should visit it in 1986 if all goes well.
Neptune: god of the sea. Neptune was discovered in 1846 on the basis of gravitational calculations. Its larger moon Triton is the largest moon in the solar system. I have heard of no recent discoveries about Neptune, and space probes won’t be visiting it anytime in the near future. (If we are lucky, Voyager 2 will visit it in 1989.)
Pluto: god of the underworld. Pluto, discovered in 1930, is the most distant known planet. Some astronomers have suggested that it was once a satellite of Neptune. Pluto is a small planet, and every time a more accurate measurement has been made, it has gotten smaller. It is now believed to be 3000 kilometers in diameter at the most (smaller than our moon). Just last year, it was discovered that Pluto has a moon, which was named Charon.
* * * * *
I always encourage astrologers to keep in touch with astronomy. The above article is really just a scattered sampling of a very lively field of knowledge. Our knowledge of the planets is increasing so rapidly because of the space program that most books about the solar system are usually at least partly out of date by the time they reach people. There are still many good books being published in the field, if you don’t mind a year or two lag. Especially appealing to me is the number of photos from space being published. If you want to keep really up to date, I strongly recommend the weekly magazine Science News—it is short and comprehensible and current.